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| United States Patent |
6,104,768
|
|
Speight
|
August 15, 2000
|
Diversity antenna system
Abstract
In order to reduce the effect of multipath propagation, diversity antennas
have been proposed in which each antenna in an array has a respective
branch tapped delay line in which the tap output signals are weighted and
summed. The sums then need to be combined in a way that improves the
signal to noise ratio. A diversity antenna system is disclosed in which
the branch weights W.sub.1s to W.sub.MS are determined for each branch by:
comparing the real part of the summed tap output signals y.sub.1 to y.sub.M
for the branch with a set level;
ascribing a symbol value of logical zero to those summed branch tap output
signals which fall to one side of the set level and ascribing a symbol
value of logical one to those summed branch tap output signals which fall
to the other side of the set level;
calculating the mean of the levels of those signals ascribed the value
logical zero and the mean of the levels of those signals ascribed the
value logical one;
calculating the distance between the level of each signal ascribed the
value logical zero and the respective mean, and the level of each signal
ascribed the value logical one and the respective mean;
calculating the sum of the distances from the means;
calculating the distances of the levels of all signals from the set level;
calculating the sum of the distances of the levels of all signals from the
set level;
calculating the ratio of the sum of the distances from the means to the sum
of the distances of the levels of all symbols from the set level; and
setting the branch weight W.sub.1S to W.sub.MS dependent on the inverse of
the ratio.
| Inventors:
|
Speight; Timothy James (Bristol, GB)
|
| Assignee:
|
Lucent Technologies Inc. (Murray Hill, NJ)
|
| Appl. No.:
|
281547 |
| Filed:
|
March 30, 1999 |
Foreign Application Priority Data
| Current U.S. Class: |
375/347 |
| Intern'l Class: |
H04L 001/02 |
| Field of Search: |
345/347,267,336
455/132
|
References Cited
U.S. Patent Documents
| 5202903 | Apr., 1993 | Okanoue | 375/347.
|
| 5349609 | Sep., 1994 | Tsujimoto | 375/100.
|
| 5530725 | Jun., 1996 | Koch | 375/347.
|
| 5724390 | Mar., 1998 | Blaker et al. | 375/229.
|
| Foreign Patent Documents |
| 93112250 | Jul., 1993 | GP | .
|
| PCT/AT97/00105 | May., 1997 | WO | .
|
Other References
Wang, H. et al "Adaptive Array Antenna Combined With Tapped Delay Line
Using Processing Gain For Direct-Sequence/Spread-Spectrum Multiple-Access
System" "Electronics & Communications in Japan, Part I, Communications
vol. 76 No. 5, May 1, 1993" pp. 101-113.
|
Primary Examiner: Ghebretinsae; Temesghen
Claims
What is claimed is:
1. A diversity antenna system, comprising an array of antennas each of
which has a respective branch tapped delay line in which the branch tap
output signals are weighted by tap weights and summed, means for weighting
each of the branch delay line sums by branch weights; and means for
summing the weighted sums, the branch weights being determined for each
branch by:
comparing the real part of the summed tap output signals for the branch
with a set level;
ascribing a symbol value of logical zero to those summed branch tap output
signals which fall to one side of the set level and ascribing a symbol
value of logical one to those summed branch tap output signals which fall
to the other side of the set level;
calculating the mean of the levels of those signals ascribed the value
logical zero and the mean of the levels of those signals ascribed the
value logical one;
calculating the distance between the level of each signal ascribed the
value logical zero and the respective mean, and the level of each signal
ascribed the value logical one and the respective mean;
calculating the sum of the distances from the means;
calculating the distances of the levels of all signals from the set level;
calculating the sum of the distances of the levels of all signals from the
set level;
calculating the ratio of the sum of the distances from the means to the sum
of the distances of the levels of all symbols from the set level; and
setting the branch weight dependent on the inverse of the ratio.
2. An antenna system as claimed in claim 1, wherein the tap weights are set
so that each branch tap delay line is a matched filter.
Description
CROSS-REFERENCE TO RELATED APPLICATION
This application claims priority of European Patent Application No.
98302654.3, which was filed on Apr. 3, 1998.
BACKGROUND OF THE INVENTION
1. Field of The Invention
This invention relates to diversity antenna systems. The background to the
invention will be explained with reference to cellular mobile telephone
systems, e.g. GSM. Other applications may occur readily to the reader.
2. Description of Related Art
Uplink communications are degraded by multipath propagation, interfering
mobile stations using the same channel and by general noise. In order to
reduce the effect of multipath propagation, diversity antennas have been
proposed in which each antenna in an array has a respective branch tapped
delay line in which the tap output signals are weighted and summed. The
sums then need to be combined in a way that improves the signal to noise
ratio. A conventional combining technique relies on estimation of the
channel response and is referred to a maximal ratio combining. The impulse
response measurements can be inaccurate leading to suboptimal combining.
SUMMARY OF THE INVENTION
In accordance with the invention, there is provided a diversity antenna
system, comprising an array of antennas each of which has a respective
branch tapped delay line in which the branch tap output signals are
weighted by tap weights and summed, means for weighting each of the branch
delay line sums by branch weights; and means for summing the weighted
sums, the branch weights being determined for each branch by:
comparing the real part of the summed tap output signals for the branch
with a set level;
ascribing a symbol value of logical zero to those summed branch tap output
signals which fall to one side of the set level and ascribing a symbol
value of logical one to those summed branch tap output signals which fall
to the other side of the set level;
calculating the mean of the levels of those signals ascribed the value
logical zero and the mean of the levels of those signals ascribed the
value logical one;
calculating the distance between the level of each signal ascribed the
value logical zero and the respective mean, and the level of each signal
ascribed the value logical one and the respective mean;
calculating the sum of the distances from the means;
calculating the distances of the levels of all signals from the set level;
calculating the sum of the distances of the levels of all signals from the
set level;
calculating the ratio of the sum of the distances from the means to the sum
of the distances of the levels of all symbols from the set level; and
setting the branch weight dependent on the inverse of the ratio.
Preferably the tap weights are set so that each branch tap delay line is a
matched filter.
BRIEF DESCRIPTION OF THE DRAWINGS
One embodiment of the invention will now be described with reference to the
accompanying drawings, in which:
FIG. 1 is a schematic drawing of an adaptive antenna system and its tapped
delay line; and
FIG. 2 is an illustrative plot of signal levels out put from the receiver
during a training sequence.
DETAILED DESCRIPTION
Referring to the drawings, each antenna 2 in an array of M antennas is
connected to a respective receiver 4. The signals received by the array
are gaussian minimum shift key (GMSK) modulated. The signals are
de-rotated to remove differential phase encoding which is applied in GMSK
modulation. The de-rotated signal from each receiver is fed to a
respective tapped delay line 6 which may be physical or simulated by one
or more processors. At each tap, the signal is weighted by individual
weights w.sub.11 to w.sub.m1 by weight setting means 8, and the weighted
signals are summed in a summer 12. The sums from each equaliser are summed
in a summer 28.
The weights in each branch are calculated by a conventional sliding
correlation technique resulting in a matched filter implementation, i.e.
the weights are set so that they are matched to the measured channel
conditions in each of the branches.
A temporary decision to ascribe a symbol value of logical zero or a logical
one depends on whether the level of the real part of the output y.sub.1 to
y.sub.m of the summer 12 is positive or negative. Thus a set decision
level 16 of zero (see FIG. 2) distinguishes the level of logical zero
symbols 18 from the level of logical one symbols 20.
The mean 22 of the distances d.sub.0 between the set level 16 and the
levels of the symbols 18 ascribe the value logical zero is calculated. The
mean 24 of the distances d.sub.1 between the set level 16 and the levels
of the symbols 20 symbols ascribed the value logical one is calculated.
The distance between the level of each symbol 18 ascribed the value logical
zero and the respective mean 22 is calculated. The distance between the
level of each symbol 24 ascribed the value logical one and the respective
mean 24 is calculated.
The sum of all the distances from the means is calculated.
The distances of the levels of all symbols from the set decision level 16
is calculated.
The sum of the distances of the levels of all symbols from the set decision
level 16 is calculated.
The ratio of the sum of the distances from the means to the sum of the
distances of the levels of all symbols from the set level is calculated
(the goodness ratio), The smaller this is, the better confidence in the
the branch response. There are m goodness ratios calculated.
The sums produced by summers 12 are themselves weighted by weights W.sub.1S
to W.sub.MS in weight setters 26 and summed in summer 28. The weights are
proportional to the inverse of the goodness ratio for the respective
branch.
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